Hydrokinetic energy — which generates power by using underwater turbines that harness moving water — is on the rise in the U.S. In January, the first U.S.-licensed, commercial, grid-connected hydrokinetic project installed the first of two 100-kW nameplate-rated turbines downriver from an existing run-of-river hydroelectric plant on the Mississippi River.
These days many renewable energy proponents are upbeat about downstream currents and their huge energy potential. Because hydrokinetic power generation relies simply on the extraction of energy from the natural velocity of water, these power systems can be placed into sources of flowing water with minimal infrastructure or environmental impacts.
Hydrokinetic energy is a new application of the concept of using water to create electricity — in this case by harnessing the natural flow or current of water. Traditional hydropower, in contrast, relies upon dams, diversionary methods, or other manmade impounding structures behind which potential energy in the water is stored.
Conventional hydropower today accounts for roughly 8% of U.S. electric output and well over 80% of U.S. renewable energy capacity (with wind, solar, geothermal, and biomass contributing the remaining 20%). As of early 2009, the only source of commercial hydrokinetic power in the U.S. is the new 200-kW nameplate-rated hydrokinetic project (expected to deliver a peak 70 kW) located in Hastings, Minn. (Figure 1).
1. Go with the flow. The Hastings hydrokinetic turbine is located behind the turbine of the existing conventional hydropower plant and uses a one-to-one turbine ratio to generate additional power from the energy remaining in the water current exiting the dam and flowing downstream. Also seen is a support barge used during installation. Courtesy: Hydro Green Energy
How Hydrokinetic Power Works
In February, POWER interviewed Wayne Krouse, the chairman and CEO of Houston-based Hydro Green Energy, about his company’s new technology, its use at the new power station near Hastings, and overall trends related to hydrokinetic energy.
"Hydro Green Energy’s hydrokinetic technology operates in river, tidal, and, potentially, ocean currents," he said. "In our Hydro+ systems, we locate the hydrokinetic turbine behind an existing conventional facility’s turbine on a one-to-one ratio to generate additional power from the energy remaining in the water current exiting the dam and flowing downstream. Placing the hydrokinetic turbines downstream from an existing hydropower plant is simply an application point for our patented technology. One way to think of this type of project is that it is the waterpower version of cogeneration: We are generating clean energy twice from the same water resource at a specific location."
Hydro Green Energy calls this type of project a Hydro+ project and has filed a number of patents on the approach; the company was the first entity to develop this method.
In an open-river setting, hydrokinetic power projects will have a capacity factor (CF) of over 99% because power will be generated by using a constant, unidirectional water current, explained Krouse. He pointed out that this is baseload power. In a tidal setting, the CF would be in the neighborhood of 30%.
At a hydro site where the existing hydropower facility operates in run-of-river mode, the CF is directly tied to the CF of the conventional hydropower plant. At a non-run-of-river hydropower plant, the CF of the hydrokinetic units may be higher than the CF of the conventional plant due to minimum flow requirements or other parameters.
The rotational speed of the company’s turbine is 21 rpm — the lowest in the hydrokinetic industry (Figure 2). The turbine blades can be stopped from spinning in approximately 8 seconds. In addition, the turbine can be pulled out of the water and placed in the service mode in under 2 minutes, leaving very little infrastructure below the waterline.
2. Hard-working water. After the second turbine is installed this summer, the Hastings hydrokinetic power station is projected to produce a maximum of 1,454 MWh of electricity annually. Courtesy: Hydro Green Energy
"The barges from which the hydrokinetic turbines are deployed (and the turbines themselves) are able to withstand wide fluctuations in water levels, substantial velocity increases, and direct impacts from large and fast-moving objects," Krouse emphasized. "The only flow parameter that would decrease power output would be a decrease in flow through the units. During extraordinary high water or flood events, the hydrokinetic arrays, which are the multi-units attached to one of the barges, could be removed from the river if necessary."
Krouse also discussed the long-term production potential of the two hydrokinetic turbines at the Hastings site.
"Power is proportional to the cube of velocity," he said. "Velocities downstream from the Hastings conventional hydropower plant vary from 3.5 m/s (11.5 ft/s) (nearest to the existing hydro plant) to 1 m/s (approximately 100 feet downstream). The units are designed for at least a 20-year lifespan. At the design point, the coefficient of performance (COP or water-to-wire efficiency) for the Hastings hydrokinetic units is 0.62, the highest in the hydrokinetic power industry at this point in time."
Using an 83% CF at nameplate capacity (100 kW per unit at 3.5 m/s), the Hastings hydrokinetic power station will produce a maximum of 1,454 MWh annually, Krouse said. Over a 20-year period, the units will achieve maximum incremental power output of 29,080 MWh. At 2.25 m/s, the hydrokinetic units together would produce approximately 508 MWh annually and 10,160 MWh over 20 years.
Hydro Green Energy is responsible for all operations and maintenance of the Hastings hydrokinetic units. These duties will be handled by the company’s engineering and project management staff, as well as local contractors.
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